In a laser oscillator for a conventional high-intensity laser system, when an ultrashort-pulse light is to be generated, the mode locking is caused for synchronizing the modes of electromagnetic waves that are resonatable in an optical resonator (oscillator), whereby the generation of an ultrashort-pulse light is enabled. For example, when a titanium-sapphire crystal that provides an extremely wide gain bandwidth for a laser is used as a laser medium, by causing the mode locking, an ultrashort-pulse light of not more than 10 fs can be generated. Since a titanium-sapphire crystal provided in an oscillator exhibits the optical Kerr effect in which a refractive index increases as a light intensity increases, the Kerr lens mode-locking method for synchronizing phases of modes by using the optical Kerr effect is used dominantly, as the mode-locking method with use of the titanium-sapphire laser.
In the conventional Kerr lens mode-locking method, a band of a pulse oscillator is determined according to, for example, a property of a titanium-sapphire crystal, though some pulse oscillators have a band higher than 10 fs. More specifically, since a laser element for obtaining a gain and a mode-locking element (mode locker) are performed by one element and a transmission-type solid substance is used for forming the element for producing a gain in an oscillator, when a wide-band pulse passes the same, a dispersion (a relative phase difference owing to a frequency difference) occurs. Accordingly, in order to generate an ultrashort pulse, that is, a transform-limited pulse, it is necessary to compensate a positive dispersion that is caused by a transmission optical medium through which the pulse passes in the resonator. For this purpose, a prism pair or a chirping mirror is disposed as a negative dispersion element in the optical resonator.
Further, as an effective method for obtaining an ultrashort-pulse high-intensity laser, the chirped pulse amplification method (CPA method) is known. Laser light has an almost uniform wavelength as compared with natural light, but still has a definite spectrum width (a wavelength distribution of waves contained in light) even though it is very small. In the CPA method, first, a time width (pulse width) of laser light outputted from a laser oscillator is expanded (pulse expansion) by using this spectrum width. This laser light is called “chirped pulse”.
The chirped pulse has a pulse height (output power) that is decreased for the foregoing expansion of the pulse width. This chirped pulse is amplified by a laser amplifier, so that an output power thereof is increased to the highest limit that barely avoids damage to a laser medium. The amplification of the chirped pulse requires a laser medium having a property of being capable of amplification even if there are differences in wavelengths to some extent (even if the pulse has a spectrum width). One principal example of the laser medium having such a property is a titanium-sapphire crystal. The laser light (chirped pulse) thus amplified to the limit of the laser medium damage is shortened in terms of time (pulse compression) by utilizing the wavelength difference. The height (output power) of the pulse is increased for the decrease in the pulse width, which results in that laser light having an ultrahigh output power higher than the limit value regarding the laser medium damage can be obtained.
Further, Patent Document 1 discloses a parametric chirped pulse amplifier device as shown in FIG. 4. This device is intended to convert a light pulse generated by a long pulse pump light source such as a diode, a fiber, or a solid laser, into a high-energy ultrashort light pulse, by using an optical parametric amplification medium. An excitation source 100 is used for generating an excitation light pulse for a predetermined duration, and is composed of an excitation diode 110 and a pulse source 120. A signal source 130 is composed of an oscillator 140 and an expander 150, and generates a signal light pulse. Both of the light pulses are coupled by optical coupling means 160, whereby a coupled light pulse is generated. A parametric amplifier 170 having a quasi-phase matching (QPM) crystal receives the coupled light pulse, and amplifies the signal light pulse with the energy of the excitation light pulse. The signal pulse thus amplified is compressed by a compressor 180. Since the parametric chirped pulse amplification is performed by the parametric amplifier 170 whereby a signal light pulse can be amplified significantly with use of an excitation light pulse having a low peak power, an ultrashort light pulse having a high energy is output from the compressor 180.
One of several important advantages of the parametric chirped pulse amplification is to enable the utilization of a long pulse pump light source. Generally, such a light source is not complicated, and is capable of supplying sufficient high energy for directly generating and amplifying an ultrashort pulse.
Patent document 1:. JP 10(1998)-268369 A